Web (server) get overloaded only by holding F5

Hi,

I have a high-performance server with high traffic and normally everything is ok (load, response, etc.) even in peak.

The problem is that anyone can overload the server only by holding f5 for several seconds.

How can I secure this? I was thinking about using 499 nginx code, but I don't know if it's possible (for example users with 5x499 in a row will get ban or something like that).

I'm already using slimits but it doesn't help.

Thanks for you help.

Tomas